1. Field of the Invention
The present invention relates to a motion compensated interframe prediction method used to encode moving pictures, and particularly to a method for interpolating motion vectors at all pixel positions in a frame on the basis of motion vectors at plural predetermined representative points to perform an interframe prediction.
2. Description of the Related Art
As a motion compensated interframe prediction method for encoding moving pictures has been known a method in which prediction signals are generated by interpolating motion vectors at all pixel positions in a frame on the basis of motion vectors at plural predetermined representative points. Examples of this method are disclosed in "Very Low Bitrate Video Coder using Warping Prediction" by T. George Campbel, Eighth Image Coding Symposium (PCSJ93) of 1993, pp8-7 (document 1), in "Motion Compensation for Video Compression using Control Grid Interpolation" by Gray J. Sullivan and Richard L. Baker, The 1991 International Conference on Acoustics, Speech and Signal Processing: ICASSP91), M9.1 (document 2), and in "A Basic Study on motion compensation with Triangles" by Yuichiro Nakaya and Hiroshi Harashima, Technical Study Report of Image Engineering Research Group of Electronic Information Communication Engineering Society, IE90-106 (document 3). According to these methods, representative points are first set up in advance. After the motion vectors at these representative points are detected, an image is sectioned into triangular or quadrilateral cells (patches) by lines through which the representative points are linked to each other, and a motion vector at a pixel position in each cell (patch) is interpolated on the basis of the motion vectors detected at the corners (apexes) of the cell by a linear interpolation method. Finally, an interframe predicted value at each pixel is determined on the basis of the motion vectors thus obtained.
According to the conventional methods as described above, since the interpolation is performed so that the motion vector varies continuously, there is an advantage that no block distortion occurs, and not only the translation, but also other various motions such as zoom, rotation, etc. can be subjected to the interframe prediction and presented to motion image. However, these methods are not applicable to a case where variation of a motion vector is sharp, for example, at a boundary portion of an object undergoing locally quick motion or different types of motion. Therefore, in such a case, a user perceives such distortion as an object that is unnaturally deformed. Particularly, a picture which moves locally quickly is visually remarkable, and thus the distortion lowers the image quality.